1. Field of the Invention
The present invention relates generally to a shift control apparatus for an automatic transmission of a motor vehicle, and more particularly to a technique for improving stability of controlling a rotating speed of a rotary member of such vehicle automatic transmission during an initial period of a shifting action of the transmission such that a rate of change in the speed of the rotary member coincides with a target value.
2. Discussion of the Prior Art
For a motor vehicle, there is available an automatic transmission having a plurality of hydraulically operated frictional coupling devices, which cooperate to selectively establish one of a plurality of operating positions (speed positions or gear positions) of the transmission. For controlling this type of automatic transmission, a shift control apparatus is known, for controlling the hydraulic pressure applied to each frictional coupling device during an engaging action thereof. This control apparatus is equipped with detecting means for detecting a rate of change of a certain parameter indicative of a progress or condition of a shifting action of the transmission. For example, the parameter may be a rotating speed or torque of a rotary member of the transmission, which speed or torque changes while the transmission is shifted. The hydraulic pressure of the frictional coupling device under control is controlled so that the detected rate of change of the parameter coincides with or nears a desired or target value. An example of such shift control apparatus is disclosed in U.S. Pat. No. 4,928,557 to Takada et al, wherein a rate of change d.omega..tau. of the rotating speed of clutch C0 during an initial period of a shifting action of the transmission is detected, so that the hydraulic pressure of the frictional coupling device is electronically controlled such that the detected rate of change d.omega..tau. coincides with the target value. The hydraulic pressure thus determined and used for an engaging action of the frictional coupling device is stored and used for a subsequent engaging action of the coupling device. For example, when the transmission is shifted from a first-speed position to a second-speed position, the duty cycle of a solenoid-operated control valve for regulating the back pressure of an accumulator for engaging brake B2 for establishing the second-speed position is calculated based on the detected rate of change d.omega..tau. and the target value d.omega..tau..sub.0, and the duty cycle used for the last engaging action of the brake B2.
In the known shift control apparatus indicated above, the parameter which indicates the progress of a shifting action of the automatic transmission, namely, an engaging action of a frictional coupling device, may be influenced by other control functions for controlling the vehicle, for instance, a suction control operation when the vehicle is provided with a so-called ACIS (Automatic Control Induction System) control function for controlling a suction control valve of an inertia supercharging engine, and a traction control operation when the vehicle is provided with a so-called TRC (Traction Control System) control function for controlling the drive force of drive wheels when the vehicle is started or accelerated. Described more particularly, if such control functions are effected while the rate of change in the above-indicated parameter is being detected, the detected value of the parameter will fluctuate and deviate from the true value. Consequently, the regulation of the hydraulic pressure of the frictional coupling devices on the basis of the detected rate of change of the parameter may cause varying or inconsistent shifting characteristic of the transmission, upon different shifting actions of the transmission, giving an uncomfortable feel to the vehicle driver. Where the vehicle has the ACIS control function, the suction control valve provided in the air intake line is opened and closed so as to maximize the engine output, depending upon whether or not the throttle valve opening is larger than a predetermined threshold and whether or not the engine speed is lower than a predetermined threshold. The opening and closing actions of this suction control valve will result in changing the speed and output torque of the engine, and thereby influencing the detected rate of change in the above-indicated parameter used for controlling the transmission. Where the vehicle has the TRC control function, the throttle valve opening, ignition timing and fuel injection amount of the engine are controlled so as to hold the slip ratio of the vehicle drive wheels within an optimum range. In this case, too, the engine speed and output torque are changed, giving an influence on the detected rate of change in the parameter.